- A
The PDU at the Transport layer is called a frame.
Why wrong: The Transport layer PDU is a segment (with TCP) or a datagram (with UDP). A frame is the PDU at the Data Link layer.
- B
Data is encapsulated with a TCP header at the Transport layer to form a segment.
The Transport layer adds a TCP header (or UDP header) to the data from upper layers, creating a segment (or datagram). This is a correct description of encapsulation.
- C
The Network layer PDU is the packet, which includes the IP header.
At the Network layer (Layer 3), the PDU is called a packet. It consists of the segment/datagram from the Transport layer encapsulated with an IP header.
- D
The Data Link layer adds a header and trailer to create a bit.
Why wrong: The Data Link layer (Layer 2) encapsulates the packet into a frame by adding a header and trailer. Bits are the Physical layer PDU.
- E
Encapsulation occurs as data moves up the OSI layers from Physical to Application.
Why wrong: Encapsulation happens when data is transmitted from source to destination; it wraps data as it moves down the layers (Application -> Presentation -> Session -> Transport -> Network -> Data Link -> Physical). Decapsulation occurs moving up.
Quick Answer
The answer is that data is encapsulated with a TCP header at the Transport layer to form a segment. This is correct because the OSI model defines specific Protocol Data Unit (PDU) names for each layer, where the Transport layer adds its header to the payload to create a segment for reliable, connection-oriented communication. On the CCNA 200-301 v2 exam, this concept tests your understanding of data encapsulation and the correct PDU names—segment, packet, and frame—across the Transport, Network, and Data Link layers respectively. A common trap is confusing the PDU at the Network layer (packet) with the Transport layer (segment), or mixing up the order of encapsulation. To remember, think of the mnemonic “Please Do Not Throw Sausage Pizza Away” for the layers, and for PDUs, recall that a “Segment” starts with ‘S’ for “Send” at Transport, a “Packet” is “Packed” with an IP header at Network, and a “Frame” is “Framed” with a MAC header at Data Link.
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which TWO statements are correct regarding Protocol Data Units (PDUs) and data encapsulation in the OSI model?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Data is encapsulated with a TCP header at the Transport layer to form a segment.
Option B is correct because at the Transport layer, the TCP protocol adds a TCP header to the payload data, forming a segment. This encapsulation process is fundamental to reliable data delivery, as the TCP header includes source and destination ports, sequence numbers, and acknowledgment numbers for connection-oriented communication.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The PDU at the Transport layer is called a frame.
- ✓
Data is encapsulated with a TCP header at the Transport layer to form a segment.
- ✓
The Network layer PDU is the packet, which includes the IP header.
Why this is correct
At the Network layer (Layer 3), the PDU is called a packet. It consists of the segment/datagram from the Transport layer encapsulated with an IP header.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The Data Link layer adds a header and trailer to create a bit.
Why it's wrong here
The Data Link layer (Layer 2) encapsulates the packet into a frame by adding a header and trailer. Bits are the Physical layer PDU.
- ✗
Encapsulation occurs as data moves up the OSI layers from Physical to Application.
Why it's wrong here
Encapsulation happens when data is transmitted from source to destination; it wraps data as it moves down the layers (Application -> Presentation -> Session -> Transport -> Network -> Data Link -> Physical). Decapsulation occurs moving up.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Data is encapsulated with a TCP header at the Transport layer to form a segment.Correct answer▾
Why this is correct
The Transport layer adds a TCP header (or UDP header) to the data from upper layers, creating a segment (or datagram). This is a correct description of encapsulation.
✗The PDU at the Transport layer is called a frame.Wrong answer — click to see why▾
Why this is wrong here
This statement incorrectly identifies the Transport layer PDU; it is actually a segment or datagram, while a frame is at Layer 2.
✗The Data Link layer adds a header and trailer to create a bit.Wrong answer — click to see why▾
Why this is wrong here
Bits are created at the Physical layer, not the Data Link layer; the Data Link layer creates frames.
✗Encapsulation occurs as data moves up the OSI layers from Physical to Application.Wrong answer — click to see why▾
Why this is wrong here
This describes decapsulation, not encapsulation. Encapsulation occurs when moving down the OSI model stack.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the direction of encapsulation (down the stack) versus de-encapsulation (up the stack), and the specific PDU names at each layer, to catch candidates who confuse these fundamental concepts.
Detailed technical explanation
How to think about this question
In the OSI model, encapsulation is a top-down process: the Application layer data is passed to the Transport layer, which adds a TCP or UDP header to form a segment or datagram. The Network layer then adds an IP header to create a packet, and the Data Link layer adds a frame header and trailer (including a Frame Check Sequence for error detection) to create a frame, which is finally converted to bits at the Physical layer. This layered approach ensures modularity and interoperability, as each layer only interacts with its adjacent layers.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Data is encapsulated with a TCP header at the Transport layer to form a segment. — Option B is correct because at the Transport layer, the TCP protocol adds a TCP header to the payload data, forming a segment. This encapsulation process is fundamental to reliable data delivery, as the TCP header includes source and destination ports, sequence numbers, and acknowledgment numbers for connection-oriented communication.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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Last reviewed: Jun 25, 2026
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